The invention relates to a low-pressure discharge lamp, having a discharge space which is closed in a vacuum-tight manner and which contains a metal vapor and a rare gas, and electrodes between which a discharge takes place during operation of the lamp; and more particularly to such a lamp, in which this discharge space is limited by two walls which are spaced apart, and a thin-walled member is arranged in the discharge space and has such a shape and dimensions that, in conjunction with the two walls, it defines a folded path for the discharge. Such a lamp is disclosed in Austrian Patent Specification 244,451.
The lamp described in the above-mentioned Patent Specification has relatively small dimensions, while the discharge path is relatively long. Constructed as, for example, a low pressure mercury vapor discharge lamp and provided with a suitable lamp base, such a lamp is suitable for use in luminaires for incandescent lamps for general illumination purposes.
In the lamp according to AT-PS 244,451 the walls which are spaced apart and enclose the discharge space are in the form of two co-axial conical glass envelopes, whose ends are sealed in a vacuum-tight manner by means of sealing glass. The thin-walled member located in the discharge space between the walls is of such a shape that during operation of the lamp a helical discharge path is created between the thin-walled body and the other walls. Discharge-tight connections are present between those walls and the member in the regions where the thin-walled member is located in the immediate vicinity of the walls. This prevents the discharge from emerging from the compartment formed by the member and one of the walls.
If a discharge were to emerge from a compartment and flash over to a neighbouring compartment, part of the discharge path would be short-circuited. The luminous flux and brightness distribution of the lamp would then be adversely affected.
In the known lamp the discharge-tight connections between the thin-walled member (which in accordance with this patent specification may consist of easily deformable material such as tin, coated with a thin glass layer) and the other walls are realised by applying, at least in the regions where the thin-walled member is located near or against the walls, a quantity of a luminescent material or a chemical material which is resistant to the action of the discharge. This requires an accurate positioning of the thin-walled member with respect to the two walls during manufacture of the lamp. Because of the specific shape of the thin-walled member, the discharge-tight connection in the known lamp extends over a relatively small area of contact between the walls and the thin-walled member. There is a risk that in the region of the connection the luminescent or chemical material detaches from the wall, for example owing to vibrations or shocks etc., so that flash-over of the discharge does in fact occur. A further disadvantage of the known lamp is that during operation dark stripes are visible on the lamp walls in the regions where the discharge-tight connections are provided between the thin-walled member and the glass wall. This is caused by the fact that the luminescent or chemical material present there hardly contributes to the production of light.